On Tue, 16 Mar 2010 08:49:34 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> On Mon, 15 Mar 2010 14:21:24 +0000
> Mel Gorman <mel@xxxxxxxxx> wrote:
>
> > On Mon, Mar 15, 2010 at 09:48:49PM +0900, Minchan Kim wrote:
> > > On Mon, 2010-03-15 at 11:28 +0000, Mel Gorman wrote:
> > > > The use after free looks like
> > > >
> > > > 1. page_mapcount(page) was zero so anon_vma was no longer reliable
> > > > 2. rcu lock taken but the anon_vma at this point can already be garbage because the
> > > > process exited
> > > > 3. call try_to_unmap, looks up tha anon_vma and locks it. This causes problems
> > > >
> > > > I thought the race would be closed but there is still a very tiny window there all
> > > > right. The following alternative should close it. What do you think?
> > > >
> > > > if (PageAnon(page)) {
> > > > rcu_read_lock();
> > > >
> > > > /*
> > > > * If the page has no mappings any more, just bail. An
> > > > * unmapped anon page is likely to be freed soon but worse,
> > > > * it's possible its anon_vma disappeared between when
> > > > * the page was isolated and when we reached here while
> > > > * the RCU lock was not held
> > > > */
> > > > if (!page_mapcount(page)) {
> > > > rcu_read_unlock();
> > > > goto uncharge;
> > > > }
> > > >
> > > > rcu_locked = 1;
> > > > anon_vma = page_anon_vma(page);
> > > > atomic_inc(&anon_vma->external_refcount);
> > > > }
> > > >
> > > > The rcu_unlock label is not used here because the reference counts were not taken in
> > > > the case where page_mapcount == 0.
> > > >
> > >
> > > Please, repost above code with your use-after-free scenario comment.
> > >
> >
> > This will be the replacement patch so.
> >
> > ==== CUT HERE ====
> > mm,migration: Do not try to migrate unmapped anonymous pages
> >
> > rmap_walk_anon() was triggering errors in memory compaction that look like
> > use-after-free errors. The problem is that between the page being isolated
> > from the LRU and rcu_read_lock() being taken, the mapcount of the page
> > dropped to 0 and the anon_vma gets freed. This can happen during memory
> > compaction if pages being migrated belong to a process that exits before
> > migration completes. Hence, the use-after-free race looks like
> >
> > 1. Page isolated for migration
> > 2. Process exits
> > 3. page_mapcount(page) drops to zero so anon_vma was no longer reliable
> > 4. unmap_and_move() takes the rcu_lock but the anon_vma is already garbage
> > 4. call try_to_unmap, looks up tha anon_vma and "locks" it but the lock
> > is garbage.
> >
> > This patch checks the mapcount after the rcu lock is taken. If the
> > mapcount is zero, the anon_vma is assumed to be freed and no further
> > action is taken.
> >
> > Signed-off-by: Mel Gorman <mel@xxxxxxxxx>
> > Acked-by: Rik van Riel <riel@xxxxxxxxxx>
>
> Reviewd-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
>
BTW, I doubt freeing anon_vma can happen even when we check mapcount.
"unmap" is 2-stage operation.
1. unmap_vmas() => modify ptes, free pages, etc.
2. free_pgtables() => free pgtables, unlink vma and free it.
Then, if migration is enough slow.
Migration(): Exit():
check mapcount
rcu_read_lock
pte_lock
replace pte with migration pte
pte_unlock
pte_lock
copy page etc... zap pte (clear pte)
pte_unlock
free_pgtables
->free vma
->free anon_vma
pte_lock
remap pte with new pfn(fail)
pte_unlock
lock anon_vma->lock # modification after free.
check list is empty
unlock anon_vma->lock
free anon_vma
rcu_read_unlock
Hmm. IIUC, anon_vma is allocated as SLAB_DESTROY_BY_RCU. Then, while
rcu_read_lock() is taken, anon_vma is anon_vma even if freed. But it
may reused as anon_vma for someone else.
(IOW, it may be reused but never pushed back to general purpose memory
until RCU grace period.)
Then, touching anon_vma->lock never cause any corruption.
Does use-after-free check for SLAB_DESTROY_BY_RCU correct behavior ?
Above case is not use-after-free. It's safe and expected sequence.
Thanks,
-Kame
> > ---
> > mm/migrate.c | 13 +++++++++++++
> > 1 files changed, 13 insertions(+), 0 deletions(-)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 98eaaf2..6eb1efe 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -603,6 +603,19 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
> > */
> > if (PageAnon(page)) {
> > rcu_read_lock();
> > +
> > + /*
> > + * If the page has no mappings any more, just bail. An
> > + * unmapped anon page is likely to be freed soon but worse,
> > + * it's possible its anon_vma disappeared between when
> > + * the page was isolated and when we reached here while
> > + * the RCU lock was not held
> > + */
> > + if (!page_mapcount(page)) {
> > + rcu_read_unlock();
> > + goto uncharge;
> > + }
> > +
> > rcu_locked = 1;
> > anon_vma = page_anon_vma(page);
> > atomic_inc(&anon_vma->migrate_refcount);
> >
>
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